1. Field of the Invention
The present invention relates to a thermal infrared solid state imaging device that detects thermal variations from incident infrared light using a two-dimensional semiconductor sensor array, and relates more particularly to a thermal infrared solid state imaging device that outputs the result of integrating electrical signals from a semiconductor sensor by means of a signal processing circuit, and to an infrared camera that uses this thermal infrared solid state imaging device.
2. Related Art
Various technologies related to thermal infrared solid state imaging devices have been disclosed.
For example, the thermal infrared solid state imaging device described in a non-patent document 1 (“A low cost 320×240 uncooled IRFPA using a conventional silicon IC process”, Ishikawa, et al., Part of the SPIE Conference on Infrared Technology and Applications XXV, April 1999, Vol. 3698, pp 556-564 (FIGS. 7 and 9)) has a gate modulated integration circuit in which the forward voltage of a diode that is constant current driven by a transistor is input to the gate of the integrating transistor. A fixed voltage Vss is connected to the source of the integrating transistor, and an integrating capacitor Cin that is periodically reset is connected to the drain. After resetting, the supply voltage is applied to the anode side of the diode of a certain row. If the temperature of the diode is changed due to the infrared light, the gate voltage of the integrating transistor Mi changes, the discharge from the integrating capacitor changes accordingly, and the voltage is read out through a sample-and-hold (S/H) circuit.
The thermal infrared solid state imaging device disclosed in a patent document 1 (JP-A-2005-241501 (FIGS. 1, 2, 7)) provides a selector switch to the gate input of a gate-modulated transistor in the integration circuit, and switches the applied voltage between the pixel voltage and the voltage of a reference pixel that does not have a heat insulation structure. A bias current supply circuit composed of a bias transistor, a switch, and a capacitor is disposed to the drain of the gate-modulated transistor, and a coupling capacitor provides AC-coupling between the integrating capacitor and the drain of the integrating transistor. An integration circuit that achieves high gain without increasing the supply voltage is achieved by the bias current supply circuit suppressing steady discharge of the integrating capacitor. In addition, by determining the bias current when the selector switch switches to the reference pixel and holding the gate bias needed to supply the bias current in the capacitor, output fluctuations caused by variation in the device temperature can be suppressed. In addition, because the gate input conversion voltage of the noise current that is passed by the integrating transistor and the bias transistor immediately before the switch turns off is stored in a capacitor in the bias current supply circuit, low frequency noise (1/f noise) that is substantially constantly during the integration period is not accumulated in the integrating capacitor, thereby helping to reduce noise.
The thermal infrared solid state imaging device taught in a patent document 2 (JP-A-2002-300475 (FIGS. 15 and 16)) performs AC-coupling between the gate of the integrating transistor (amplification transistor) and the pixel area by means of a first coupling capacitor. A sampling transistor is disposed between the gate and the drain of the integrating transistor. The drain of the integrating transistor is connected to a power supply through a reset transistor, and is connected through a control switch and a second coupling capacitor to an integrating capacitor that is periodically clamped by a first switch. A sample and hold circuit composed of a second switch and a capacitor is connected to the downstream side of the integrating capacitor. A reference pixel row that does not have a heat insulation structure is disposed to the last line of the pixel area. In the first half of one horizontal period the drain of the integrating transistor is reset. The second switch is then turned on and the threshold voltage of the integrating transistor is held on the gate side of the first coupling capacitor. The second switch then opens and the reference pixel line is input in the following period. The first switch is left on at this time. As a result, a signal voltage denoting the variation from the threshold voltage is held by the second coupling capacitor. By then opening the first switch and selecting a pixel row, only signals denoting a different temperature than the reference pixel are accumulated in the integrating capacitor. This invention can thus suppress deviation in the threshold voltage of the integrating transistor, reset noise that is accumulated on the gate side of the first coupling capacitor when the second switch is open, and output fluctuations caused by device temperature fluctuations. The second coupling capacitor also functions to suppress the 1/f noise of the integrating transistor and the power supply transistor.
Some problems with the thermal infrared solid state imaging device taught in the non-patent document 1 are described below.                (1) Low voltage drive is difficult because the potential of the integrating capacitor Cin is discharged from the reset voltage Vref during the integration period even in the incident infrared state used as the reference. This problem is particularly pronounced because discharge increases if the bias current is increased to increase sensitivity and increase the gain of the integration circuit, or if the integration time is increased to reduce noise.        (2) The source voltage Vss of the integrating transistor is necessary. Because the current of the integrating transistor depends on the difference between the gate voltage and the source voltage, variation in the source voltage Vss causes the output voltage of the integration circuit to change.        (3) The pixel output voltage changes with change in the ambient temperature. This change cannot be differentiated from the change in incident infrared light.        
In the case of the solid state imaging device shown in FIG. 7 in the patent document 1, the voltage on the pixel side of the switch is equal to the power supply voltage minus the forward voltage of the diode and the voltage drop between the drive line and the signal line. The voltage drop is low at the left side near the power supply node and increases to the right side. This voltage drop depends on the current flow and the resistance determined by the pixel pitch and the line width of the drive line. When the line resistance is 0.5Ω per pixel, the pixel current is 10 μA, and the horizontal pixel count is 320, there is a voltage drop of 260 mV between the left end and the right end of the drive line. This voltage is usually a vale that cannot be ignored compared with the threshold voltage of the integrating transistor. Because the mutual conductance of the integrating transistor that determines the gain of the integration circuit is proportional to the (gate voltage−threshold voltage), the gain distribution of the integration circuit occurs horizontally across the pixel area and appears as on the screen as uneven sensitivity. This problem can be reduced by such measures as setting the threshold voltage sufficiently high, reducing the pixel current, or reducing the drive line resistance, but the sensitivity deviation cannot be reduced to zero. There is also a voltage effect distribution on the signal line. But this is not a problem because the voltage drop on the signal line plus the voltage drop on the power supply line can be held constant on all of the vertical lines by setting the power supply line resistance on the drain side of the selector switch connected to the drive line to 1/(horizontal pixel count) of the resistance of the signal line in the pixel area.
Some problems with the thermal infrared solid state imaging device taught in the patent document 2 are described next.                (1) An unexposed pixel reading period and an exposed pixel reading period must be provided in the horizontal scanning period. Both periods must be equal so that the coupling capacitor can be subtracted after integrating both signals. Because the horizontal scanning period is usually determined by the television format, the signal integration time is less than or equal to half the horizontal scanning period. The integration circuit gain that is proportional to the integration time therefore drops, the noise bandwidth that is proportional to the integration time increases, and noise is greater than conventionally.        (2) While suppressing signal variation due to threshold voltage variation in the integrating transistor is an important feature, a long time is required for sampling the threshold voltage of the integrating transistor because the current of the integrating transistor tends to be cut-off.        
“A Novel Noise Reduction Technique for the Uncooled Infrared Image Sensor with Bulk-micromachined Pixels,” a report on using this method presented at the 12th International Display Workshop/Asia Display 2005 (IDW/AD'05), reported that the 20 μsec duration of one sampling period was shown to be too short, and thus multiple samples were required, recommending five or more samplings. with a normal sensor, it is preferable to set the integration period to the approximately 50 μsec duration of one horizontal scanning period and reset the capacitor in the approximately 10 μsec of the horizontal blanking period. Because current is supplied to the pixels at each horizontal line, the noise bandwidth of the pixels can be reduced the greatest and the SNR of the sensor can be improved. The prior art thus cannot sample the threshold voltage during the horizontal blanking period, and the benefit of sampling during this period cannot be achieved.
The present invention is directed to solving the foregoing problems, and an object of the invention is to provide a thermal infrared solid state imaging device, as well as an infrared camera using the thermal infrared solid state imaging device, that can easily achieve high gain without increasing the power supply voltage, does not require a bias voltage sensitive to gain and the output voltage, has little change in output or uneven image sensitivity due to ambient temperature variation, and can effectively reduce noise including 1/f noise.